Our basic proposal remains the same, namely to measure the changes in pain sensitivity in humans that occurs both during and following noxious heating or noxious mechanical stimulation of the skin. We will continue to correlate these results with recordings of activity evoked by the same stimuli in single A- and C-fiber nociceptive afferents innervating the monkey hand. We will study those stimulus conditions that lead either to an augmented pain response (hyperalgesia) and possible sensitization of fiber response or to a reduction of pain (suppression and adaptation) and fatigue of fiber response. In addition, we now propose to overcome inherent shortcomings in this cross-species correlative approach by percutaneous recording of nociceptive afferent activity in the peripheral nerve of alert human subjects while they are performing the sensory psychophysical tasks. Nociceptor responses recorded from monkeys and from humans will be directly correlated with psychophysical functions that relate intensive and temporal judgements of pain to stimulus temperature or to certain biomechanical events during thermal and mechanical stimulations respectively. The results obtained will not only provide direct evidence for peripheral neural coding mchanisms of pain in humans but will also further test the validity of using subhuman primates as a model for physiological studies of human pain.